feat(forms): country→timezone autoset, "Other" channel hint, polish

Client form: when nationality is picked and timezone empty, primary
IANA zone for the country is pre-filled (skips when user has chosen
a zone explicitly). When a contact's preferred channel is `'other'`,
the inline `Label` field flips to "Specify" / "e.g. Telegram, Signal"
so the rep records what the channel actually is.

Yacht form: replace the free-text 2-letter flag input with the shared
`CountryCombobox` so flags stay valid ISO codes.

User settings: timezone pre-populates from
`Intl.DateTimeFormat().resolvedOptions().timeZone` on first load
(was empty before); country change auto-fills timezone with the same
helper as the client form. Phone field upgraded to the shared
`<PhoneInput>` (country-flag dropdown + AsYouType formatter) seeded
from the page's country state.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/yachts/yacht-form.tsx

@@ -19,6 +19,7 @@ import {
 } from '@/components/ui/select';
 import { Sheet, SheetContent, SheetHeader, SheetTitle, SheetFooter } from '@/components/ui/sheet';
 import { Separator } from '@/components/ui/separator';
+import { CountryCombobox } from '@/components/shared/country-combobox';
 import { OwnerPicker, type OwnerRef } from '@/components/shared/owner-picker';
 import { TagPicker } from '@/components/shared/tag-picker';
 import { apiFetch } from '@/lib/api/client';
@@ -185,7 +186,12 @@ export function YachtForm({ open, onOpenChange, yacht, initialOwner }: YachtForm
 
               <div className="space-y-1">
                 <Label>Flag</Label>
-                <Input {...register('flag')} placeholder="e.g. MT" />
+                <CountryCombobox
+                  value={watch('flag') ?? null}
+                  onChange={(iso) => setValue('flag', iso ?? undefined)}
+                  placeholder="Select flag country…"
+                  clearable
+                />
               </div>
 
               <div className="space-y-1">